Devolatilization of resins (removal of volatile components in resins) and recovery of gas emissions are both important processes in olefin polymerization, which have been studied in numerous documents. For example, CN 102171256 describes a process and method of resin devolatilization, while in U.S. Pat. No. 5,391,656, it discloses a method for recovering a compressed and condensed gas emission.
Gas emissions generated in producing olefin polymers are gas streams which contain reactant gases discharged from a reactor, a flash tank, a devolatilizer, or the like. For example, gas emissions generated from the serous-polyethylene production process mainly come from flash tanks or devolatilizers, while gas emissions generated from the gas phase process are mainly from devolatilizers. As processes and production grades vary, the reactant gas comprises various kinds of hydrocarbons (including monomers, comonomers, condensing agents, solvents, reaction byproducts, alkane impurities, etc.), inert gases, other small molecular substances, and the like. Inert gasses generally include nitrogen and saturated hydrocarbons. The former is useful in balancing pressures in the reactor or in transporting catalysts, while the latter enters the reaction system as impurities together with the raw materials. Small molecular substances, e.g., usually hydrogen, are used for adjusting molecular weight of olefin polymers. For example, in the linear low density polyethylene (LLDPE) process for producing ethylene-butylene copolymers, the reactant gasses contain ethylene (C2H4), butene (n-C4H8), ethane (C2H6), butane (n-C4H10), hydrogen (H2), isopentane (i-C5H12), nitrogen (N2), etc. Resins and reactant gasses discharged from the reactor are fed to the resin devolatilizer together, so as to remove hydrocarbons carried by or dissolved in the resins. In some processes, nitrogen is used for carrying solid resins from the reaction system to the devolatilizer, for which reference can be made to U.S. Pat. No. 4,372,758.
The operating pressure in a devolatilizer is far less than that in the reaction system, such that the moment a resin enters the devolatilizer, a majority of hydrocarbons carried would be rapidly separated therefrom. After entering the devolatilizer from an upper portion thereof, the resin would flow downward in a dense-phase flow with a certain material level maintained. In order to remove the hydrocarbons dissolved in the resin, inert gases are fed into the devolatilizer from a middle portion and a lower portion thereof, usually nitrogen as a sweep gas, which is mainly used for further lowering the partial pressure of the hydrocarbons. The sweep gas flows from bottom to top and removes the residual hydrocarbons in interacting with the resin. The resin devolatilization is performed in order to secure operations in downstream devices for granulation, air feeding, etc., facilitate storage and transport of polyolefin products, and reduce peculiar smells of the products as well.
Gases discharged from a top portion of the devolatilizer are referred to as gas emissions, which comprise all the reactant gasses separated from the resin in the devolatilizer and the aforementioned sweep gas. Obviously, the gas emissions contain a huge amount of hydrocarbons, which would cause severe material wastes, economic losses, and environment pollutions in case of direct vent or discharge into a flare system without being recycled. Therefore, recycling of hydrocarbons in gas emissions is of significant importance.
Gas emissions generally contain a low content of hydrocarbons, and therefore cannot be directly recycled to the reaction system or other process devices for use, so that a device for recovering the gas emissions is usually necessary. The device for recovering gas emissions, substantially a separation device of hydrocarbons and sweep gasses, on the one hand, concentrates the hydrocarbons and then feeds the concentrated hydrocarbons to the reaction system or other process devices, and on the other hand, concentrates the sweep gasses and then feeds the concentrated sweep gasses to a devolatilizer for reuse. However, gas emissions with low pressure and low concentration in hydrocarbons are rather difficult to be recycled.
The compression refrigeration process, a traditional gas separation process, has been widely used for recovering gas emissions generated in producing polyolefins owning to simple process steps and large handling capacity thereof. The process steps of compression refrigeration are as follows. The gas emissions are first fed into a low pressure cooler. Where the cooler has an outlet temperature lower than the dew-point of the gas emissions, some heavy hydrocarbon components would be condensed, so that the gas emissions would enter a low-pressure condensate tank as a gas-liquid mixture and condensed liquids are recovered therein. In order to further recover hydrocarbons, the non-condensed gas is first compressed by a compressor for improving dew-points thereof and then cooled and condensed respectively by a high-pressure cooler and a high-pressure condenser. Next, a gas-liquid mixture enters a high-pressure condensate tank for gas-liquid separation. The separated condensate and the low-pressure condensate can both be fed back to the reaction system for reuse, while the gas is discharged to the flare. Obviously, the compression refrigeration process can be used for recovering hydrocarbon components (condensable hydrocarbons) heavier than sweep gasses. However, as discussed above, the gas emissions contain merely a small amount of condensable hydrocarbons, and therefore a large amount of non-condensable gasses are compressed and cooled in vain, thus reducing the economy of the compression refrigerating process. As can be seen, the compression refrigeration process mainly has the following disadvantages: 1) the lower the content of condensable hydrocarbons in the gas emissions is, the lower the efficiency of the compression refrigeration process will be; 2) C1 to C3 hydrocarbons with the boiling points thereof close to the boiling point of the sweep gas (nitrogen) are difficult to be recovered, usually with a recovery rate of not more than 30%; and 3) the concentration of the sweep gas in the non-condensed gas emissions fails to reach the requirements for devolatilization and therefore can only be discharged to the flare system due to its incapacity of being recycled.
U.S. Pat. No. 5,521,264 discloses a process for recovering unreacted monomers by physical absorption and desorption. The process comprises treating a gas emission by a compression refrigeration system, and recovering a condensate while a non-condensed gas emission entering an absorption column; absorbing hydrocarbons in the non-condensed gas emission by an absorbent in the absorption column to obtain a top gas stream containing nitrogen and light components, and a bottom liquid stream containing the absorbent and monomers that are absorbed, wherein the top gas stream is optionally discharged to a flare, to serve as a transport gas, or fed to a degassing position, while the bottom liquid stream enters a desorption column; and separating the absorbent and monomers in the desorption column to obtain a top stream containing monomers and a bottom stream containing the absorbent, wherein the bottom stream returns to the absorption column for reuse while the monomers return to the reaction system. Besides, U.S. Pat. No. 5,681,908 improves the process as disclosed in U.S. Pat. No. 5,521,264 with an addition of a separation process of byproducts so as to prevent enrichment of the byproducts in the system. Although the absorption-desorption process enables further recovery of hydrocarbons on the basis of the compression refrigeration process, it largely raises operating and investment costs due to complex process steps, large quantity of devices, and additional energy consumption and utilities in repeated heating and cooling of a large number of absorbents.
CN 200920203363.X discloses a process for recovering propylene in the production of polypropylene, wherein the gas emissions generated in the production passes through a dust filter, a buffer gas cabinet, a compressor, and a condenser to generate a gas-liquid mixture, which is fed into a gas-liquid separator for gas-liquid separation. A non-condensable gas obtained therein, after filtration and heat tracing, enters a membrane separator, while hydrocarbon components such as propylene enrich in a membrane permeation side and return to an inlet of the compressor. In the above application, the recovery rate of propylene is improved to 99% by the combination of the compression refrigeration and membrane separation processes. However, as pointed out in CN 87103695, a gas adequately pure for direct reuse cannot be obtained by separation performed in the membrane system. To solve the above problem, the membrane separation system has to be combined with other processes (e.g. a process combined with a pressure swing absorption device as disclosed in CN 87103695) for further gas purification. Alternately, repeat compressions in the compressor can be carried out as proposed in CN 200920203363.X. The above two processes can both greatly increase investments and energy consumption.
CN 202485331 discloses a process for recovering gas emissions generated in the membrane separation. In the process, a cryogenic process is performed for further recovering hydrocarbons in view of the membrane system. In the cryogenic process, a pressure of a tail gas does work through a turboexpander, so as to achieve the low temperature required in the condensation of light components such as ethylene. However, this process cannot be satisfactorily combined with the devolatilization of polyolefin processes. For example, the tail gas generated in the process cannot be further recycled due to too low a pressure thereof. Meanwhile, where there is a high content of hydrogen, recycling of a sweep gas would result in enrichment of hydrogen in the system, thus causing a series of problems such as reduction of the recovery rate.
To conclude the above, although the processes and devices for recovering gas emissions generated in the production of olefin polymers in the prior art are rather distinctive. None of the processes or devices can achieve high-efficient recovery and recycling of sweep gasses.